Theses and Dissertations at Montana State University (MSU)
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Item Structural analysis of the Gaurishankar Lineament to understand its role as a cross fault in the east-central Nepal Himalaya(Montana State University - Bozeman, College of Letters & Science, 2023) Baral, Nischal; Chairperson, Graduate Committee: Mary S. HubbardThe Gaurishankar Lineament (GL) in east-central Nepal is a topographic feature in eastern Nepal that has been proposed to be a cross-fault (faults perpendicular to the regional E-W strike). This feature has also been proposed to have blocked the lateral propagation of the thrust rupture that was activated during the Mw 7.8 Gorkha earthquake. To understand whether the GL is a cross fault, I conducted a field study along ~E-W traverses south of Gaurishankar peak in the Rolwaling (Greater Himalayan Sequence (GHS)) and in the central Tamakoshi and Jiri regions (Lesser Himalayan Sequence (LHS)). I augmented my fieldwork with petrographic and kinematic analysis of oriented samples. As circumstantial evidence for the GL representing the topographic expression of a cross fault, I identified multiple steep fracture patterns orthogonal to regional E-W striking fabric in the Rolwaling region and an additional system of NW striking fabric orthogonal to the NNE trending GL. In the Jiri region, the offset in a thin band of graphitic schist within the rocks of the LHS aligns along the GL and may represent part of a cross-fault system. An eastward plunging synclinal fold that covers the broad region between Tamakoshi and Jiri is disrupted to the east along the GL, also consistent with the presence of a cross fault. Several NW striking faults in the Jiri region end along the GL, as suggested by the topographic expression from the satellite imagery. Further study is needed to recognize and develop a comprehensive understanding of the GL as a cross fault and to explore a newly identified system of NW striking fault.Item Reconstructing the age, provenance, and thermal history of the basal Great Valley forearc basin, northern California(Montana State University - Bozeman, College of Letters & Science, 2022) Romero, Mariah Christina; Chairperson, Graduate Committee: Devon A. Orme; This is a manuscript style paper that includes co-authored chapters.Forearc basins are important sediment archives of Earth's geologic history, preserving a record of the erosional history of magmatic arcs, subduction zone dynamics, and climatic changes over millions of years. However, questions remain about the early developmental stages of a forearc basin, including the relationship between a forearc basin and its underlying basement, and thermal histories of exhumed, ancient forearc basins that preserve extensive sedimentary successions. This dissertation examines the basement underlying the Great Valley Forearc basin, the upper Coast Range Ophiolite and ophiolitic breccia, and Great Valley Group strata using sedimentology, sandstone modal petrography, geochronology, thermochronology, and cathodoluminescence, photoluminescence, and Raman spectroscopy. Chapter 1 evaluates Upper Jurassic-Lower Cretaceous strata and underlying Coast Range Ophiolite in the northwestern Sacramento basin to constrain the timing of initial deposition within the Great Valley Forearc, identify potential provenances, and provide a tectonic model for the early development of the forearc. Detrital zircon and petrographic data from a localized breccia interval directly underlying basal forearc strata indicate provenance from the Coast Range Ophiolite and North American margin, with formation ongoing by ~151 Ma. Detrital zircon data from Upper Jurassic-Lower Cretaceous strata yield maximum depositional ages between ~165-141 Ma and are interpreted to reflect diachronous deposition in segmented depocenters during the early development of the forearc that was proximal to the Sierra Nevada-Klamath magmatic. Chapter 2 utilizes apatite and zircon (U-Th)/He thermochronology to constrain the thermal history of the Sacramento basin, which includes documenting minimum burial temperatures for the western outcrop belt to have exceeded 85°C with cooling in the Cenozoic, whereas the subsurface strata of the central-eastern parts of the basin reached ~180-200°C with cooling in the middle-Late Cretaceous and Cenozoic. Chapter 3 examines gabbro and granitic basement rock underlying the Great Valley Forearc, along with several zircon U-Pb age standards, to demonstrate that radiation damage in zircon can non-destructively be estimated using cathodoluminescence spectroscopy, a complementary technique to photoluminescence and Raman spectroscopy. This dissertation highlights the significance of studying forearc basins along with their accompanying basement component, as the linked relationship is crucial for understanding how forearc basins evolve.